The inventive subject matter relates to power systems and, more particularly, to island grid power supply systems and methods.
In many industrial applications, the magnitude of the load is greater than the capacity of a single generator. For these applications, multiple generators may be paralleled to match the load requirements and/or to provide redundancy in the event of a generator failure. These generators that are paralleled may be of the same size and type. In some applications, such as in island grid applications, a utility source may not be present to support load transients or stabilize the load voltage and frequency. Therefore, the generators may change frequency and voltage due to a load transient.
Natural gas is becoming a more desirable fuel for power generation than diesel because of lower fuel cost and improved emissions. However, drawbacks of natural gas engine generators sets include a reduced step response capability and poor control performance with light loads. The magnitude of a load step change that can be tolerated by a generator may be dependent on current loading and the degree of change in load. A typical target for frequency accuracy for marine, industrial and residential loads is 5%. If the load requires a 5% frequency regulation, a natural gas engine generator set may be capable of accepting only a 10% load change. In contrast, a diesel engine generator set may accept a 50% load change with the same regulation. Accordingly, natural gas generators may be severely limited in load step capability to maintain a similar output characteristic to that of diesel generators.
In practice, paralleled generators tend to be all the same size, which may create a problem for the operator if the load is time varying. For example, a typical application may have 3 generators supplying the load. When the load is light, the generators may be loaded artificially with resistor banks so the generators have the minimum load required by the manufacturer. Since the generators have a generally poor response time, the artificial load may also be used to get the generator at speed prior to load application. When the load step is applied, the artificial load is turned off. The use of this artificial load allows the generators to maintain acceptable load voltage and frequency for load steps but at a significant cost in fuel consumption. Accordingly, there is a need for improved techniques of operating such systems.